1. Field of the Invention
The present invention relates to an intermediate transfer member for use in an image forming apparatus employing an electrophotographic system, such as an electrophotographic copier, a printer, a facsimile, or a device combining these machines, and to an image forming apparatus having the same, and more particularly to an intermediate transfer member used in an image forming apparatus for obtaining a reproduced image by being transferred a toner image formed on a latent image carrier once onto an intermediate transfer member, and the image then being transferred onto paper or another transfer material, and to an image forming apparatus having the same.
2. Description of the Related Art
In an image forming apparatus to which the electrophotographic system is applied, a uniform electric charge is formed on a latent image carrier made of inorganic or organic photoconductive photosensitive material. An electrostatic latent image is formed by a laser with modulated image signals or by light emitting diodes. Thereafter, the electrostatic latent image is developed by charged toner, and a visible toner image is formed. The toner image is then electrostatically transferred onto a transfer material such as a recording paper directly or by way of an intermediate transfer member, so that a desired reproduced image is obtained. In particular, in a known intermediate transfer system, the toner image formed on the image carrier is transferred onto the intermediate transfer member in a primary transfer, and the toner image on the intermediate transfer member is transferred onto the recording paper in a secondary transfer.
The material for the endless belt used in the image forming apparatus employing such an intermediate transfer system is polycarbonate resin (PC), polyvinylidene fluoride (PVDF), polyalkylene phthalate, a blend material of PC and polyalkylene phthalate (PAT), ethylene tetrafluoroethylene copolymer (ETFE), or another thermoplastic resin, and a semiconductive endless belt made therefrom has been proposed.
In Japanese Patent No. 2560727 and Japanese Patent Application Laid-Open (JP-A) No.5-77252, an ordinary carbon black is dispersed as a conductive fine powder in polyimide resin, and intermediate transfer belts made therefrom are proposed. When ordinary carbon black is dispersed as a conductive fine powder in polyimide resin, variations in electric resistance depending on environmental changes of temperature and humidity are low, but it is difficult to disperse carbon black uniformly, and fluctuations of electric resistance within a plane tend to be larger. Further, since the resistance depends very much on the voltage, in the transfer section, the applied voltage may be concentrated, and the electric resistance may be lowered due to transfer voltage.
In the above-described prior art, in order to obtain a transfer image of high image quality, the electric resistance value of the intermediate transfer member must be controlled to be within a prescribed range, the in-plane fluctuations of the intermediate transfer member (the difference between maximum and minimum values of resistance) must be small, and electric resistance must not change greatly even if the environmental conditions change. In long-term durability tests, a small variation in electric resistance is required for always obtaining a transfer image of high quality.
Also in the prior art, when ordinary carbon black is dispersed in the polyimide resin as a conductive fine powder, secondary aggregation occurs, and a chain of conductivity is likely to be formed, and in-plane fluctuations of electric resistance tend to increase. As a result, due to large in-plane fluctuations of electric resistance, in the transfer section, the applied voltage is concentrated, and the electric resistance is lowered due to applied voltage.
Specifically, for example, in a case of using postcards or similar sheets shorter than the width of the intermediate transfer member, after transferring more than 1000 sheets continuously, when a half-tone (magenta 30%) image was transferred, the paper running area was left in blank. This image defect of blank portions was particularly notable in the low temperature and low humidity environment of 10xc2x0 C. and 15% RH. Blanking of the paper running area is caused when peeling discharge occurs between the intermediate member and paper when peeling off the paper, due to the applied voltage in the secondary transfer section. The surface resistivity of the paper running area of the intermediate transfer member becomes lower than that of the peripheral area, and the transfer efficiency is lower than that of the peripheral area. It is assumed that, as voltage is applied repeatedly, the carbon black contained in the belt is oxidized, and the conductivity is raised, thereby lowering the resistance.
The decline in resistance tended to be large when the dispersion of the carbon black contained in the belt was poor.
Low resistance leads to shortening of life of the intermediate transfer member, which causes to increase the maintenance labor and running costs, and hence is undesirable.
It is hence an object of the present invention to provide an intermediate transfer member which has little deformation caused by driving, in which a decline in resistance due to transfer voltage is suppressed, and which has improved uniformity of electric resistance, low dependence on electric field, and few variations in resistance due to the environment, and to provide an image forming apparatus capable of obtaining transfer images of high quality stably for a long period.
Such object is achieved by the following means.
A first aspect of the present invention is an intermediate transfer member having a polyimide resin film containing oxidized carbon black.
A second aspect of the present invention is an image forming apparatus comprising an intermediate transfer member having a polyimide resin film containing oxidized carbon black.